Step 1: Using 123D Design Online

I decided to use 123D Design Online from Autodesk, for three main reasons:

It is easy to use - designed from scratch to do simple operations well

It maintains an object "history", so any feature can be edited later - this allows for customization

It runs in a browser - no need to install software or keep it updated

Starting 123D Design Online is simple, but you must be using a browser that supports WebGL (for now that means Apple's Safari, Google Chrome and recent versions of Mozilla Firefox.) If you have one of those:

Go to http://www.123dapp.com/

In the Apps menu, select 123D Design

If this is the first time you've used 123D Design, you'll be prompted to install a plugin

You should also create an Autodesk account, or sign on with any of several social networks

Step 2: Adding geometry and joining parts

Adding some basic shapes to scene is easy - drag and drop from the parts bin on the right of the screen.

In this case, add a cylinder, make it 20mm wide and long, and 5mm high. Also add a cube, 20x20x4mm.

You should be able to drag them together, and side of the square will snap to the middle of the cylinder.